1. Field of the Invention
This invention relates to aluminum alloys, and particularly to aluminum alloys used in corrosive environments.
2. Description of the Relevant Art
Aluminum has long been known for its resistance to corrosion, a property which renders it a favorable material of construction for a wide variety of purposes. In particular, it has gained wide acceptance in the manufacture of heat exchangers such as, for example, automotive radiators and evaporators for air conditioning units. Depending on the type of exchanger, the parts may be joined together by welding or brazing.
Despite aluminum's inherent corrosion resistance, corrosion still occurs. This is particularly true in materials joined by brazing, since the brazing operation causes intergranular penetration of the core material by certain species in the brazing alloy, notably silicon. When the brazed product is subjected to a corrosive environment, the intergranular regions in the core, where silicon is present in high concentrations, are particularly susceptible to corrosion. In automotive radiators, for example, where the brazing alloy is present only on the exterior surface of the tubes, the salts and moisture from the road are sufficiently corrosive to cause attack from the outside, resulting ultimately in pinhole formation.
This problem has been addressed in the literature in a variety of ways. An early example is Miller, U.S. Pat. No. 2,821,014 (Jan. 28, 1958), where it is disclosed that intergranular corrosion problems in flux and dip brazing are alleviated by adding an interlayer between the structural member portion and the brazing layer. The interlayer is aluminum or an aluminum-base alloy, particularly certain magnesium-containing alloys, having a melting point greater than that of the structural alloy. The solution offered by Singleton et al., U.S. Pat. No. 3,788,824 (Jan. 29, 1974) and its divisional, U.S. Pat. No. 3,881,879 (May 6, 1975), is directed to vacuum brazing, and involves the addition of iron to either the core alloy or the cladding alloy as an alloying element, resulting in improvements in both corrosion resistance and sag resistance.
Anthony et al., U.S. Pat. No. 4,039,298 (Aug. 2, 1977) address both flux and vacuum brazing, and disclose a composite of complex and highly specified composition as being particularly beneficial in terms of corrosion properties. The disclosed core alloy contains specified amounts of manganese, copper, chromium, silicon and iron as alloying elements with both a solid solution and an alpha-phase, whereas the alloying elements in the cladding are bismuth and silicon. An additional disclosure by the same patentees appears in U.S. Pat. No. 4,093,782 (June 6, 1978) and its continuation-in-part, U.S. Pat. No. 4,167,410 (Sept. 11, 1979), in which the core alloy contains a specified combination of chromium and manganese, with resultant improvements in both corrosion resistance and sag resistance.
A still further disclosure by the same patentees appears in U.S. Pat. No. 4,209,059 (June 24, 1980), where a conventional core alloy is clad with a brazing alloy on one side and a "sacrifical cladding layer" on the other, the result being a lessening of crevice corrosion between the aluminum header plate and the plastic tank in an automobile radiator. A "sacrificial anode" effect is disclosed in Tanabe et al., U.S. Pat. No. 4,317,484(March 2, 1982), and Terai et al., U.S. Pat. No. 4,203,490 (May 20, 1980), for tube-and-fin heat exchangers by incorporating tin and zinc into the fin core material and manganese into the tube material. A similar differentiation between fins and tubes is disclosed in Kanada et al., U.S. Pat. No. 4,410,036 (Oct. 18, 1983), whereby the fins are provided with a lower electrochemical potential.
Setzer et al., U.S. Pat. No. 4,399,695 (Nov. 30, 1976), disclose a core alloy which contains a chromium-manganese-zirconium combination, the sole claimed benefit however being an improvement in sag resistance. Sag resistance is also addressed by Toma et al. in U.S. Pat. No. 4,511,632 (Apr. 16, 1985), where manganese, silicon and zinc are included in the cladding layer. A combination of copper and titanium as primary alloying elements in the core alloy is disclosed in Kaifu et al., U.S. Pat. No. 4,339,510 (July 13, 1982), as providing a benefit in intergranular corrosion resistance.
A different approach is disclosed by Nakamura, U.S. Pat. No. 4,172,548 (Oct. 30, 1979), in which corrosion following fluxless brazing processes in general (including both vacuum brazing and brazing in an inert atmosphere) is controlled by controlling the grain size of the brazing sheet to at least 60 microns in diameter, achieved by a controlled cold work followed by a full anneal.
Thus, with the exception of Nakamura, existing solutions generally involve the introduction of specific elements in the alloy compositions. Processing modifications have also been used to similar effect, notably that disclosed in copending, commonly owned application Ser. No. 634,529, filed July 26, 1984, now U.S. Pat. No. 4,586,964. In general, however, such features as specific combinations, degrees and sequences of strain hardening and annealing are generally used for controlling the ductility and tensile properties of the final product. Setzer et al., referenced above, demonstrates several of these combinations, ranging from those ending with a fully hardened product (maximum cold work) to those ending with a fully strainfree (annealed) product. The use of a partial anneal as the final step to leave the desired amount of cold work remaining in the product is disclosed by Singleton, U.S. Pat. No. 3,963,454 (June 15, 1976) at column 4, lines 34-58.